1. Field of the Invention
The present invention relates to an improved method of removing methane from subterranean coal deposits.
The inclusion of methane in coal beds has long been a safety problem in many areas of the world. The methane is tightly absorbed in the coal micropores and on the coal surfaces, and is released during mining, creating a safety hazard.
2. Brief Description of the Prior Art
There have been many attempts to overcome the problem of methane in coal deposits in the past. Early attempts to overcome the problem involved drilling a series of vent holes in the coal deposit in the hope that the methane would flow from the coal deposit out the vent holes. Later attempts included such things as applying vacuum to the coal deposit to accelerate methane removal, and additionally attempts have been made to displace the methane by passing a displacing fluid such as gas or water through the deposit. All of these attempts have been successful in some degree, but they have not been completely satisfactory due to the inadequate removal of methane obtained by these processes and also due to the excessive time required to carry out these processes. The rate of advance in working up coal seams has been greatly increased with the advent of mechanization of underground coal mining. With the more rapidly advancing working face, the release of methane gas from the coal and the surrounding rock occurs constantly due to release of rock pressure and crack formations connected therewith. For this reason, in order to maintain adequate safety standards, operation of mechanized coal mines must be interrupted from time to time while steps are taken to maintain the concentration of methane gas below the permissible maximum. The interruption of the mining operation is undesirable for both technical and economic reasons.
U.S. Pat. No. 3,384,416 issued to Ruehl describes a method for fracturing and degassing of coal seams, utilizing a volatile low-boiling liquid for injection into the coal seam to form a gaseous mixture of volatilized liquid and mine gas and thereafterward withdrawing the thus formed gaseous mixture through the injection well from the thus fractured zone to relieve the latter of mine gas. Ruehl teaches carbon dioxide as being a suitable volatile low-boiling liquid for use in his method. However, the method taught by Ruehl suffers in that long periods of time are required to remove significant amounts of methane gas from the coal, and the area affected by the treatment is limited to the immediate vicinity of the injection well. There has been a continuing need for a faster and more efficient method of removing methane from coal deposits.